When drilling boreholes into formations in the earth's subsurface it is desirable to obtain information related to the nature and structure of the formations penetrated by the borehole. To this end, many different tools have been developed to measure (log) certain physical properties of the borehole and surrounding formations. For example, the depth location, borehole size, hydrocarbon pore volume, porosity, lithology, and permeability of a subsurface formation are often deduced from measurable quantities during drilling such as electrical resistivity, density, photoelectric factor (Pe), hydrogen index, natural (spontaneous) radioactivity, acoustic velocity, nuclear magnetic resonance, and thermal neutron capture cross section (Sigma), among others.
Logging tools typically carry a source that emits energetic radiation into the formation and one or more detectors that can sense the resulting interactions of the radiation. Detected signal data are typically transmitted uphole, temporarily stored downhole for later processing, or combined in both techniques, to evaluate the formation from which the data were gathered.
One technique for formation logging uses gamma ray density probes, which are devices that incorporate a gamma ray source and a gamma ray detector, often shielded from each other to prevent the detector from counting gamma radiation emanated directly from the source. During the operation of the probe, gamma rays (photons) are emitted from the source and enter the formation to be studied. In the formation, they interact with the atomic electrons of the material of the formation by either photoelectric absorption, by Compton Scattering, or by pair production. In photoelectric absorption as well as pair production, the photon involved in the interaction is removed, but in pair production two lower-energy photons are formed.
In Compton scattering, the involved photon loses some of its energy while changing its original direction of travel, the loss of energy being a function of the scattering angle. Some of the photons emitted from the source into the formation material are accordingly scattered back toward the detector. Many scattered photons do not reach the detector, because their direction is again changed by a second Compton scattering, or they are thereafter absorbed by the photoelectric absorption process or the pair production process. The scattered photons which reach the detector and interact with it are counted by electronic counting equipment associated with the detector to produce count rates (e.g., counts per unit time). The resulting data are then used to infer the bulk density of the formation, which can be used to determine the formation's porosity.
Another technique for logging uses neutrons. Here, a either a chemical or a pulsed neutron source emits neutrons into the formation and thermal and/or epithermal neutron detectors measure the neutron flux at several distances from the neutron source. The neutron flux depends on the properties of the formations through which the neutrons pass in traveling from source to detector. Neutrons lose kinetic energy through inelastic and elastic scattering, and neutrons—particularly neutrons that have slowed down or ‘thermalized’—can be absorbed by the nuclei of formation atoms. The neutron slowing down time measured by one or more of the detectors via count rates is a shallow measurement of hydrogen index, which can be used to determine the formation's solid composition, porosity, and type of saturating fluid. Traditional porosity measurements rely on deriving liquid-filled porosity from the ratio of the neutron fluxes from at least two different distances from the source.
Difficulties encountered in neutron porosity and gamma ray density measurements include the disturbing effects of undesired interfering materials located between the probe and the formation sample, such as drilling fluid and mud cake on the borehole wall. Drilling fluid invasion is a process that occurs in a well being drilled with higher wellbore pressure (for example, caused by large mud weights) than formation pressure. The liquid component of the drilling fluid (i.e., mud filtrate) continues to “invade” the porous and permeable formation until the solids present in the mud (for example, bentonite or barite) clog enough pores to form a mudcake capable of preventing further invasion.
Invasion also has significant implications for well logging. In some cases, the “depth of investigation” of a well logging tool is only a few inches, and it is possible that drilling fluid has invaded beyond this depth. Therefore the readings that are taken that are influenced by the formation fluids are measuring mud filtrate properties rather than formation (in situ) properties, or, in some cases, a combination of mud filtrate properties and formation properties. In the case of Sigma measurements in particular, the invasion of mud filtrate into the formation can mask the actual nuclear properties of in-situ hydrocarbons and water.
Logging probes have tried to compensate for the effect of mudcake density and mudcake thickness on formation density, and hence on porosity measurements, by including two or more detectors axially spaced along the borehole at different distances from the source of radiation. The near or short spaced detector is for receiving radiation which has scattered mainly in the materials near the borehole wall, including the mudcake. The far or long-spaced detector(s) is for receiving radiation which has scattered principally in the formation. The difference in time-decay response is associated with a number of factors, including the solid composition, fluid composition, presence/absence of invasion, and porosity.
Still, most active gamma logging systems require complex collimation schemes to narrowly define either the beam of radiation emanating from the source to direct it into a specific region of the formation or the beam of radiation received back by the detector to ensure that only radiation back-scattered from a particular region of the formation was detected, or both. With these schemes, it is presupposed that the region of interaction between the radiation and the formation can be narrowly defined and restricted to a small region. Not only is precise collimation of radiation beams difficult to accomplish, but the assumption that a collimated beam only interacts with a precisely definable portion of the formation surrounding the borehole is erroneous.
Given these and other difficulties with well logging operations, there is a need for improved accuracy and contrast when determining a property of a formation. Indeed, a large contrast in Sigma (i.e., the nuclear capture cross-section) is necessary to quantify the effect of filtrate invasion, i.e., salty mud filtrate with large concentrations of Cl− invading low-absorption media such as hydrocarbon-bearing rocks. The disclosed methods address these and other needs.